As a solid-state device which operates and oscillates in a frequency band of terahertz waves, an oscillator using a negative resistance element, such as a resonant tunneling diode (RTD), has been proposed. In particular, an oscillator using an RTD takes advantage of an electromagnetic wave gain based on the intersubband transition of electrons in the semiconductor quantum well structure of the RTD, and room-temperature oscillation in a terahertz band has been reported. Japanese Patent Application Laid-Open No. 2007-124250 and Jpn. J. Appl. Phys., Vol. 47, No. 6, pp. 4375-4384, 2008 disclose an oscillator in which a double-barrier RTD and a planar slot antenna are integrated on a semiconductor substrate. The oscillator oscillates at room temperature in a terahertz band under a bias voltage at which differential negative resistance appears in the current-voltage characteristics of the RTD.
An oscillator using a negative resistance element, such as an RTD, is known to cause low-frequency parasitic oscillation attributable to a bias circuit including a power supply. Parasitic oscillation can be a cause for degradation in the oscillation output of a desired resonance frequency in a frequency band of terahertz waves. To take measures against this problem, IEEE MICROWAVE AND GUIDED WAVELETTERS, VOL. 5, NO. 7, JULY 1995 pp. 219-221 discloses a technique for locating a stabilization circuit between a bias power supply and an RTD. The stabilization circuit is comprised of a resistor and a capacitor disposed in parallel with the RTD, and sets a resonance circuit to low impedance in all frequencies except a resonance frequency. In addition, the stabilization circuit is located in a position within a distance of λ/4 (λ is the wavelength of a desired resonance frequency in a terahertz-band frequency domain) from the RTD. With these contrivances, parasitic oscillation is suppressed to realize room-temperature oscillation in a terahertz band with an oscillator using a negative resistance element.